This patent application claims priority under the Paris Convention of Japanese Patent Application, Hesei 10, Patent Application No. 139580, naming as inventor Yongzhang Huang, filed May 21, 1998 with the Japanese Patent Office in the name of Sumitomo Eaton Nova, on behalf of the inventor.
The present invention pertains to a deceleration electrode configuration for use in an ion implanter, and its method of operation. The deceleration electrode can be directly utilized for a high-current, ultra-low energy ion implanter to enable a high-current ion beam to be generated with ultra-low energy and with no energy contamination.
Ion implanters are used to fabricate semiconductor devices. Typically, they generate positively charged ions such as boron (B+) and phosphorous (P+) from an ion source. The positively charged ions are extracted from the ion source and formed into an ion beam.
The ion beam undergoes a mass separation process in which utilizes differences in the mass and electric charge of the beam components to select only ions of appropriate charge-to-mass ration. These ions in the beam are then accelerated to achieve an energy sufficient to allow implantation into a wafer. The beam is transported along a beam passage to a wafer or wafers positioned in an implantation chamber. The ions in the beam are then implanted into the wafer or wafers.
In a prior art ion implantation apparatus, integrated circuits may be produced by selective implantation using masks and inactive layers. An apparatus for this implantation technology is large, complex and expensive. Moreover, traditional ion implanters, which operate in energy ranges exceeding one kilo-electron-volt (KeV), have limited capability with respect to low-energy ion implantation processes which are required for ultra-shallow junctions on such integrated circuits.
High current, ultra-low energy ion implantation processes are required in order to form ultra-shallow junctions in the integrated circuits. Therefore, it is necessary to expand the operating energy range of the implanter from energies of about 1 kilo-electron-volt (KeV) to a few hundred electron-volts (eV) for a commercial, most modern high-current ion implantation apparatus.
In order to obtain a high current at an ultra low energy, an ion beam having a relatively high energy is extracted from the ion source, then undergoes a mass analysis, and travels close to the proximity of a wafer where the ion beam is decelerated. An apparatus for the deceleration of an ion beam is called a deceleration (or decel) electrode. Then, the decelerated ion beam travels to a target such as a wafer.
It is contemplated that an effective high-current, ultra-low energy ion implanter will necessarily include a deceleration electrode. However, the deceleration electrode configuration for a high energy, ultra-low energy implanter is difficult to achieve for two reasons: energy contamination of the ion beam and beam xe2x80x9cblow-upxe2x80x9d caused by the xe2x80x9cspace charge forcexe2x80x9d.
First energy contamination caused by high energy neutral particles in the ion beam, present a problem. Harmful neutral particles are produced in the region upstream of the deceleration electrode by the collisions among ions and background residual gas molecules. A portion of the ions become neutralized, thereby forming neutral particles. These neutral particles will pass through the deceleration electrode without being decelerated and will become implanted into the wafer due to their retained energy. The deceleration electrode will, however, reduce the energy of the positive ions passing therethrough prior to implantation. Because the neutral particles have energy that is greater than the decelerated ions, they will be implanted deeper into the wafer.
This phenomenon of energy contamination and is an intrinsic problem in using deceleration electrodes. When the beam energy prior to deceleration is increased in order to increase the beam current, the energy contamination problem becomes more severe. The ratio of the beam energy before the deceleration to after deceleration is referred to as the deceleration (decel) ratio.
Separating neutral particles out of the ion beam may principally eliminate the energy contamination problem. In prior art medium current ion implanters, which operate at higher energies than high-current, ultra-low energy implanters, deceleration electrodes that are oriented perpendicular to the ion beam axis accomplish deceleration of the ion beam. Neutral particles can thereby pass through the deceleration electrodes.
Neutral particle separation devices are utilized upstream of the deceleration electrodes to eliminate energy contamination. One separation mechanism is an electrical deflector wherein a transverse electric field bends the ion beam but exerts no effect on the neutral particles, which are separated after passing through the deflector. However, such a device is not acceptable for use in a high-current, ultra-low energy implanter because the neutralization of the ion beam is destroyed, exaggerating beam blow-up and preventing focused transport over a long distance. Also, the space required by the electrical deflector adds to the distance required to be traveled by the ion beam. Another separation mechanism is a magnet wherein a transverse magnetic field bends the ion beam and separates the ion beam form the neutral particles. Although the magnet does not adversely affect the beam neutralization, it is still too long to be used effectively for a high current, ultra-low energy beam.
The second problem that the present invention addresses is that of the xe2x80x9cspace charge forcexe2x80x9d. Positively (like charged) ions which form the ion beam repulse each other because of the so-called xe2x80x9cspace-charge forcexe2x80x9d, which becomes more dominant at lower energies. The space charge force causes the ion beam to spread or xe2x80x9cblow-upxe2x80x9d. Because of the space-charge force, the lateral spread of an ion beam is proportional to:
(m/q)xc3x97(Iz2/U3/2)xe2x80x83xe2x80x83(Eq. 1)
wherein it is assumed that the ion beam is uniform and has a circular cross section. In the above equation , m is an ion mass, q is an ion charge, I is a beam current, U is beam energy, and z is the travelling distance of the ion beam. As is obvious from the above, a shorter ion beam travelling distance is better for a larger current.
The space-charge force becomes larger, and therefore extremely problematic, for lower ion beam energies. Thus, if an ion beam travels over a long distance to a wafer, it becomes more difficult for all ions to reach the wafer. Hence, it is necessary to keep the distance short between a deceleration apparatus and a wafer. Shortening the distance is very important in order to obtain a focused ion beam with a large current and ultra low energy. In order to resolve the second issue, it is necessary to make a separation device, which separates neutral particles from an ion beam, as compact as possible.
Thus, it is an object of the present invention to provide a deceleration for an ion implanter which can separate neutral particles out of an ion beam and, simultaneously, decelerate the positive ions in the beam prior to implantation. It is a further object to provide such a deceleration electrode that enables the formation of an energy contamination-free ultra-low energy ion beam with higher currents than that which have been previously achievable. It is a further object to provide such a deceleration electrode in compact form so as to minimize the effect of space charge force on the beam.
A deceleration electrode for a high-energy, ultra-low ion implanter is provided. The deceleration electrodes are xe2x80x9ctiltedxe2x80x9d (i.e., not perpendicular with respect the ion beam axis. The deceleration electrode reduces the energy of the ion beam and simultaneously separates neutral particles out of the ion beam. The length of the deceleration electrode is slightly longer than a conventional deceleration electrode. However, because the device functions to also separate neutral particles out of the ion beam, the need for a separate neutral particle separation device is eliminated. Thus, the compact design of the dual function electrode configuration permits a shortening of the distance that a high-current, ultra-low energy ion beam must travel to the target wafer. Further, because the neutral particles can be almost completely separated from the ion beam, the decel ratio may be set high enabling an ultra-low energy, high current ion beam.
In the tilted decel apparatus of the present invention, a plurality of decel electrodes having openings are arranged in an inclined manner against the axis which is perpendicular to the base axis of a beam passage. An-ion -beam impinging with a predetermined ion beam offset distance and inclination angle passes through gaps formed between the decel electrodes and thus, an ion beam is decelerated and neutral particles are separated from the ion beam. Thus, an ion beam can be formed having a high current, low neutral contamination, and ultra-low energy.